Tiny gold clusters—just two atomic layers thick and roughly 0.5 nm in diameter—are far more active room-temperature CO oxidation catalysts than microscopic gold clusters of many other sizes and thicknesses, according to a study (Science 2008, 321, 1331). The investigation provides new details about the high catalytic activity exhibited by nanoparticles of gold, a metal that in bulk form is inert. An international research team led by Christopher J. Kiely of Lehigh University and Graham J. Hutchings of Cardiff University, in Wales, used traditional chemical methods to disperse gold particles on iron oxide supports and subjected the samples to various heat treatments. The procedures yielded a series of samples with widely varying catalytic activities and particle sizes ranging from individual atoms to one- and two-layer clusters several nanometers in diameter. On the basis of state-of-the-art microscopy and other analytical methods, the group concludes that the high catalytic activity can be attributed primarily to 0.5-nm bilayer clusters containing approximately seven to 10 gold atoms.